The present invention relates to heat-removing circuit boards having a core of metallic materials and an electrically insulating coating.
Fiberglass-reinforced epoxy-resin circuit boards provided for semiconductor technology and electronics, as well as circuit boards of highly heat-conductive metals with insulating varnish or plastic layers or aluminum-oxide ceramic circuit boards, frequently can no longer meet the specified requirements in use, particularly with respect to mechanical strength, heat removal or flammability.
Cooling or heat removal are possible only inadequately and expensively by superficial forced air cooling of the electronic modules on fiberglass-reinforced epoxy-resin circuit boards by means of air blowers with flow velocities, which widely differ locally, of maximally, at first 3 m/sec and later, even 6 m/sec. Also water-cooled flat assemblies for removing the Joule heat (dissipation losses) in order to keep the components (which may give off as much as 2 W) below 70.degree. C., are not satisfactory with respect to reliability and operating safety, because an even brief and local failure of the cooling system can lead to irreversible damage to the operability of the electronic system or even to the failure of the overall installation.
While the relatively expensive aluminum-oxide ceramic plates are usable up to circuit board formats of about 25.times.50 mm as conductor run substrates, their use is very limited for larger formats, especially in view of the insufficient mechanical strength (brittleness) and lack of flatness. The use of larger circuit boards of aluminum oxide ceramic is usually prevented by the costs.
Tests with plates, with and without holes, of highly heat-conductive metals with insulating varnish or plastic layers also have shown numerous shortcomings. Thus, the varnish layers are too thin at the edges of the holes, small holes are clogged up with varnish or plastic, and, when plastic powder layers are sprinkled-on, the layers become too thick and droop at the holes in funnel-fashion instead of at right angles. Because of the particularly low heat conductivity of insulating varnishes and plastics, thicker layers have an especially deterimental effect on local heat removal processes. To all this must, finally, be added the large problems of adhesion of the conductor run metal layers on varnish or plastic surfaces.